This application discloses an acidic crystalline catalyst which is surface inactivated by exposure to high pressure coking conditions. The application further discloses uses for such catalyst, including a process for producing high molecular weight hydrocarbons having unexpectedly low methyl branching from a lower olefin feedstock. Such products are useful as distillate fuels, lubricants and chemical intermediates.
Recent work in the field of olefin upgrading has resulted in a catalytic process for converting lower olefins to heavier hydrocarbons. Heavy distillate and lubricant range hydrocarbons can be synthesized over shape-selective catalysts such as ZSM-5 at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Such a technique has been developed by Garwood, et al, as disclosed in European Patent Application No. 83301391.5, published Sep. 29, 1983. In U.S. Pat. Nos. 4,150,062; 4,211,640; 4,227,992; and 4,547,613 Garwood, et al disclose operating conditions for a process for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10.sup.+ product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of non-reactive paraffins and the like being acceptable components. Lower C.sub.6 -C.sub.9 oligomer products may be recycled.
Shape-selective oligomerization, as it applies to the conversion of C.sub.2 -C.sub.10 olefins over ZSM-5, may produce higher olefins up to C.sub.30 and higher. As reported by Garwood in "Intrazeolite Chemistry 23", (Amer. Chem. Soc., 1983), reaction conditions favoring higher molecular weight product are low temperature (200.degree.-260.degree. C.), elevated pressure (about 2000 kPa or greater), and long contact time (less than 1 WHSV). The reaction under these conditions proceeds through the acid-catalyzed steps of (1) oligomerization, (2) isomerization-cracking to a mixture of intermediate carbon number olefins, and (3) interpolymerization to give a continuous boiling product containing all carbon numbers. The channel systems of medium pore catalysts impose shape-selective constraints on the configuration of the large molecules, accounting for the differences with other catalysts.
The desired oligomerization-polymerization products include C.sub.10.sup.+ substantially linear aliphatic hydrocarbons. This catalytic path for propylene feed provides a long chain which generally has lower alkyl (e.g., methyl) substituents along the straight chain.
The final molecular configuration is influenced by the pore structure of the catalyst. For the higher carbon numbers, the structure is primarily a methyl-branched straight olefinic chain, with the maximum cross-section of the chain limited by the dimension of the largest zeolite pore. Although emphasis is placed on the normal 1-alkenes as feedstocks, other lower olefins, such as 2-butene or isobutylene, are readily employed as starting materials due to rapid isomerization over the acidic zeolite catalysts.
The viscosity index of a hydrocarbon lube oil can be related to its molecular configuration. Extensive branching in a molecule usually results in a low viscosity index. Similarly, extensive branching can be problematic in distillate fractions which are utilized as diesel fuels. Such branching increases auto ignition delay beyond that desired for effective operation of diesel engines. Cetane number (CN) is a fuel property which is measured in standard test engines in which the compression ratio is variable (ASTM-D613-65). High CN fuels ignite at low compression ratio relative to accepted standard fuels. It is known that for paraffinic compounds of a given carbon number, the cetane number is highest for the normal paraffin and decreases with increased methyl branching. Cetane number (CN) is a measurable quantity which inversely reflects the extent of branching in the components of a diesel fuel. Minimally branched internal olefins, e.g., C.sub.12.sup.=, may have utility as decene extenders (chemical intermediates) in synthetic lube oil production.
It is believed that two modes of oligomerization/polymerization of olefins can take place over shape-selective acidic zeolites, such as HZSM-5. One reaction sequence takes place at Bronsted acid sites inside the channels or pores, producing essentially linear materials. The other reaction sequence occurs on the outer surface, producing more branched material. By decreasing the surface acid activity of such zeolites, reduced methyl-branching occurs, resulting in products of higher VI and higher cetane number.
Several techniques may be used to increase the relative ratio of intra-crystalline acid sites to surface active sites in acidic porous crystalline materials. This ratio tends to increase with crystal size due to geometric relationship between volume and external surface area.
It is known to use certain basic materials to deactivate the Bronsted acid sites on the surface of aluminosilicate catalysts. U.S. Pat. No. 4,520,221 and U.S. Pat. No. 4,568,786, Chen, et al., which are expressly incorporated herein disclose bulky amines, such as di-tert-butyl pyridine, as such basic materials. U.S. Pat. No. 4,870,038 to Page et al., incorporated herein by reference, discloses olefin oligomerization using a zeolitic catalyst (ZSM-23) wherein the zeolite surface is rendered substantially inactive for acidic reactions by neutralizing with a bulky pyridine compound, e.g., 2,4,6-collidine. However, these techniques generally require continuous addition of the basic material to the feedstock and operation is limited to lower temperatures in order to avoid loss of the basic material from the catalyst, and low acid activity resulting in the need for operation at low space velocity.
Deposition of carbonaceous materials by coke formation can also shift the effective ratio of intra-crystalline acid sites to surface active sites, as disclosed in U.S. Pat. No. 4,547,613, wherein a zeolite catalyst is conditioned by contact with C.sub.2-16 olefin at 400.degree. to 1000.degree. F. at 0 to 100 psig for 1-70 hours. The conditioned catalyst provides an oligomerized olefin product having a high viscosity index. Other examples of coke-selectivation of zeolite catalysts are set out in U.S. Pat. Nos. 4,001,346 to Chu et al. and 4,128,592 to Kaeding et al. All of the foregoing references are incorporated herein by reference.